System and method for high-performance, low-power data center interconnect fabric

ABSTRACT

A system and method are provided that support a routing using a tree-like or graph topology that supports multiple links per node, where each link is designated as an Up, Down, or Lateral link, or both, within the topology. The system may use a segmented MAC architecture which may have a method of re-purposing MAC IP addresses for inside MACs and outside MACs, and leveraging what would normally be the physical signaling for the MAC to feed into the switch.

PRIORITY CLAIMS/RELATED APPLICATIONS

This patent application claims the benefit under 35 USC 119(e) to U.S. Provisional Patent Application Ser. No. 61/256,723 filed on Oct. 30, 2009 and entitled “System and Method for Enhanced Communications in a Multi-Processor System of a Chip (SOC), which is incorporated herein by reference.

FIELD

The disclosure relates generally to a switching fabric for a computer-based system.

BACKGROUND

With the continued growth of the internet, web-based companies and systems and the proliferation of computers, there are numerous data centers that house multiple server computers in a location that is temperature controlled and can be externally managed as is well known.

FIGS. 1A and 1B show a classic data center network aggregation as is currently well known. FIG. 1A shows a diagrammatical view of a typical network data center architecture 100 wherein top level switches 101 a-n are at the tops of racks 102 a-n filled with blade servers 107 a-n interspersed with local routers 103 a-f. Additional storage routers and core switches. 105 a-b and additional rack units 108 a-n contain additional servers 104 e-k and routers 106 a-g FIG. 1 b shows an exemplary physical view 110 of a system with peripheral servers 111 a-bn arranged around edge router systems 112 a-h, which are placed around centrally located core switching systems 113. Typically such an aggregation 110 has 1-Gb Ethernet from the rack servers to their top of rack switches, and often 10 Gb Ethernet ports to the edge and core routers.

However, what is needed is a system and method for packet switching functionality focused on network aggregation that reduces size and power requirements of typical systems while reducing cost all at the same time and it is to this end that the disclosure is directed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a typical data center system;

FIG. 2 is an overview of a network aggregation system;

FIG. 3 illustrates an overview of an exemplary data center in a rack system;

FIG. 4 illustrates a high-level topology of a network aggregating system;

FIG. 5A illustrates a block diagram of an exemplary switch of the network aggregation system;

FIG. 5B illustrates the MAC address encoding;

FIG. 6 illustrates a first embodiment of a broadcast mechanism of the network aggregation system;

FIG. 7 illustrates an example of unicast routing of the network aggregation system;

FIG. 8 illustrates an example of fault-resistant unicast routing of the network aggregation system; and

FIG. 9 illustrates a second embodiment of a broadcast mechanism of the network aggregation system.

DETAILED DESCRIPTION OF ONE OR MORE EMBODIMENTS

The disclosure is particularly applicable to a network aggregation system and method as illustrated and described below and it is in this context that the disclosure will be described. It will be appreciated, however, that the system and method has greater utility since the system and method can be implemented using other elements and architectures that are within the scope of the disclosure and the disclosure is not limited to the illustrative embodiments described below.

The system and method also supports a routing using a tree-like or graph topology that supports multiple links per node, where each link is designated as an Up, Down, or Lateral link, or both, within the topology. In addition, each node in the system maybe be a combination computational/switch node, or just a switch node, and input/output (I/O) can reside on any node as described below in more detail. The system may also provide a system with a segmented Ethernet Media Access Control (MAC) architecture which may have a method of re-purposing MAC IP addresses for inside MACs and outside MACs, and leveraging what would normally be the physical signaling for the MAC to feed into the switch. The system may also provide a method of non-spoofing communication, as well as a method of fault-resilient broadcasting, which may have a method of unicast misrouting for fault resilience. In the context of network security, a spoofing attack is a situation in which one person or program successfully masquerades as another by falsifying data and thereby gaining an illegitimate advantage.

The system may also provide a rigorous security between the management processors, such that management processors can “trust” one another. In the example system shown in FIG. 5A (which is described below in more detail), there is a management processor within each SoC (the M3 microcontroller, block 906, FIG. 5A). The software running on the management processor is trusted because a) the vendor (in this case Smooth-Stone) has developed and verified the code, b) non-vendor code is not allowed to run on the processor.

Maintaining a Trust relationship between the management processors allow them to communicate commands (e.g. reboot another node) or request sensitive information from another node without worrying that a user could spoof the request and gain access to information or control of the system.

The system may also provide a network proxy that has an integrated microcontroller in an always-on power domain within a system on a chip (SOC) that can take over network proxying for the larger onboard processor, and which may apply to a subtree. The system also provide a multi-domaining technique that can dramatically expand the size of a routable fat tree like structure with only trivial changes to the routing header and the routing table.

FIG. 2 illustrates a network aggregation system 300. The network aggregation supports one or more high speed links 301 (thick lines), such as a 10-Gb/sec Ethernet communication, that connect an aggregation router 302 and one or more racks 303, such as three racks 303 a-c as shown in FIG. 3. In a first rack 303 a, the network aggregation system provides multiple high-speed 10 Gb paths, represented by thick lines, between one or more Smooth-Stone computing unit 306 a-d, such as server computers, on shelves within a rack. Further details of each Smooth-Stone computing unit are described in more detail in U.S. Provisional Patent Application Ser. No. 61/256,723 filed on Oct. 30, 2009 and entitled “System and Method for Enhanced Communications in a Multi-Processor System of a Chip (SOC)” which is incorporated herein in its entirety by reference. An embedded switch 306 a-d in the Smooth-Stone computing units can replace a top-of-rack switch, thus saving a dramatic amount of power and cost, while still providing a 10 Gb Ethernet port to the aggregation router 302. The network aggregation system switching fabric can integrate traditional Ethernet (1 Gb or 10 Gb) into the XAUI fabric, and the Smooth-Stone computing units can act as a top of rack switch for third-party Ethernet connected servers.

A middle rack 303 b illustrates another configuration of a rack in the network aggregation system in which one or more Smooth-Stone computing units 306 e, f can integrate into existing data center racks that already contain a top-of-rack switch 308 a. In this case, the IT group can continue to have their other computing units connected via 1 Gb Ethernet up to the existing top-of-rack switch and the internal Smooth-Stone computing units can be connected via 10 Gb XAUI fabric and they can integrate up to the existing top-of-rack switch with either a 1 Gb or 10 Gb Ethernet interconnects as shown in FIG. 2. A third rack 303 c illustrates a current way that data center racks are traditionally deployed. The thin red lines in the third rack 303 c represent 1 Gb Ethernet. Thus, the current deployments of data center racks is traditionally 1 Gb Ethernet up to the top-of-rack switch 308 b, and then 10 Gb (thick red line 301) out from the top of rack switch to the aggregation router. Note that all servers are present in an unknown quantity, while they are pictured here in finite quantities for purposes of clarity and simplicity. Also, using the enhanced SS servers, no additional routers are needed, as they operate their own XAUI switching fabric, discussed below.

FIG. 3 shows an overview of an exemplary “data center in a rack” 400 according to one embodiment of the system. The “data center in a rack” 400 may have 10-Gb Ethernet PHY 401 a-n and 1-Gb private Ethernet PHY 402. Large computers (power servers) 403 a-n support search; data mining; indexing; Apache Hadoop, a Java software framework; MapReduce, a software framework introduced by Google to support distributed computing on large data sets on clusters of computers; cloud applications; etc. Computers (servers) 404 a-n with local flash and/or solid-state disk (SSD) support search, MySQL, CDN, software-as-a-service (SaaS), cloud applications, etc. A single, large, slow-speed fan 405 augments the convection cooling of the vertically mounted servers above it. Data center 400 has an array 406 of hard disks, e.g., in a Just a Bunch of Disks (JBOD) configuration, and, optionally, Smooth-Stone computing units in a disk form factor (for example, the green boxes in arrays 406 and 407), optionally acting as disk controllers. Hard disk servers or SS disk servers may be used for web servers, user applications, and cloud applications, etc. Also shown are an array 407 of storage servers and historic servers 408 a, b (any size, any vendor) with standard Ethernet interfaces for legacy applications.

The data center in a rack 400 uses a proprietary system interconnect approach that dramatically reduces power and wires and enables heterogeneous systems, integrating existing Ethernet-based servers and enabling legacy applications. In one aspect, a complete server or storage server is put in a disk or SSD form factor, with 8-16 SATA interfaces with 4 ServerNodes™ and 8 PCIe x4 interfaces with 4 ServerNodes™. It supports disk and/or SSD+ServerNode™, using a proprietary board paired with a disk(s) and supporting Web server, user applications, cloud applications, disk caching, etc.

The Smooth-Stone XAUI system interconnect reduces power, wires and the size of the rack. There is no need for high powered, expensive Ethernet switches and high-power Ethernet Phys on the individual servers. It dramatically reduces cables (cable complexity, costs, significant source of failures). It also enables a heterogeneous server mixture inside the rack, supporting any equipment that uses Ethernet or SATA or PCIe. It can be integrated into the system interconnect.

The herein presented aspects of a server-on-a-chip (SOC) with packet switch functionality are focused on network aggregation. The SOC is not a fully functionally equivalent to an industry-standard network switch, such as, for example, a Cisco switch or router. But for certain applications discussed throughout this document, it offers a better price/performance ratio as well as a power/performance ratio. It contains a layer 2 packet switch, with routing based on source/destination MAC addresses. It further supports virtual local area network (VLAN), with configurable VLAN filtering on domain incoming packets to minimize unnecessary traffic in a domain. The embedded MACs within the SOC do have complete VLAN support providing VLAN capability to the overall SOC without the embedded switch explicitly having VLAN support. It can also wake up the system by management processor notifying the management processor on link state transitions to reprogram routing configurations to route around faults. Such functionality does not require layer3 (or above) processing (i.e., it is not a router). It also does not offer complete VLAN support, support for QoS/CoS, address learning, filtering, spanning tree protocol (STP), etc.

FIG. 4 shows a high-level topology 800 of the network system that illustrates XAUI connected SoC nodes connected by the switching fabric. The 10 Gb Ethernet ports Eth0 801 a and Eth1 801 b come from the top of the tree. Ovals 802 a-n are Smooth-Stone nodes that comprise both computational processors as well as the embedded switch. The nodes have five XAUI links connected to the internal switch. The switching layers use all five XAUI links for switching. Level 0 leaf nodes 802 d, e (i.e., N0 n nodes, or Nxy, where x=level and y=item number) only use one XAUI link to attach to the interconnect, leaving four high-speed ports that can be used as XAUI, 10 Gb Ethernet, PCIe, SATA, etc., for attachment to I/O. The vast majority of trees and fat trees have active nodes only as leaf nodes, and the other nodes are pure switching nodes. This approach makes routing much more straightforward. Topology 800 has the flexibility to permit every node to be a combination computational and switch node, or just a switch node. Most tree-type implementations have I/O on the leaf nodes, but topology 800 let the I/O be on any node. In general, placing the Ethernet at the top of the tree minimizes the average number of hops to the Ethernet.

In more detail, the ovals shown in the tree-oriented topology in FIG. 6 represent independent nodes within a computing cluster. FIG. 5A illustrates one example implementation of an individual node of the cluster. When looking at a conventional implementation of a topology e.g. in FIG. 6, usually computing nodes are found in the lower level leaf nodes (e.g. N00-N08), and the upper level nodes don't have computing elements but are just network switching elements (N10-N21). With the node architecture shown in FIG. 6A, the A9 Cores (905) may be optionally enabled, or could be just left powered-off. So the upper level switching nodes (N10-N21) in FIG. 6 can be used as pure switching elements (like traditional implementations), or we can power on the A9 Cores module and use them as complete nodes within the computing cluster.

The switch architecture calls for a routing frame to be prepended to the Ethernet frame. The switch operates only against fields within the routing frame, and does not inspect the Ethernet frame directly. FIG. 5 a shows a block diagram of an exemplary switch 900 according to one aspect of the system and method disclosed herein. It has four areas of interest 910 a-d. Area 910 a corresponds to Ethernet packets between the CPUs and the inside MACs. Area 910 b corresponds to Ethernet frames at the Ethernet physical interface at the inside MACs, that contains the preamble, start of frame, and inter-frame gap fields. Area 910 c corresponds to Ethernet frames at the Ethernet physical interface at the outside MAC, that contains the preamble, start of frame, and inter-frame gap fields. Area 910 d corresponds to Ethernet packets between the processor of routing header 901 and outside MAC 904. This segmented MAC architecture is asymmetric. The inside MACs have the Ethernet physical signaling interface into the routing header processor, and the outside MAC has an Ethernet packet interface into the routing header processor. Thus the MAC IP is re-purposed for inside MACs and outside MACs, and what would normally be the physical signaling for the MAC to feed into the switch is leveraged. MAC configuration is such that the operating system device drivers of A9 cores 905 manage and control inside Eth0 MAC 902 and inside ETH1 MAC 903. The device driver of management processor 906 manages and controls Inside Eth2 MAC 907. Outside Eth MAC 904 is not controlled by a device driver. MAC 904 is configured in Promiscuous mode to pass all frames without any filtering for network monitoring. Initialization of this MAC is coordinated between the hardware instantiation of the MAC and any other necessary management processor initialization. Outside Eth MAC 904 registers are visible to both A9 905 and management processor 906 address maps. Interrupts for Outside Eth MAC 904 are routable to either the A9 or management processor. The XGMAC supports several interruptible events that the CPUs may want to monitor, including any change in XGMII link fault status, hot-plugging or removal of PHY, alive status or link status change, and any RMON counter reaching a value equal to the threshold register.

In some cases, there may be Preamble, Start of Frame, and Inter-Frame gap fields across XAUI, depending on the specific micro-architecture. The routing frame header processor may standardize these fields. The XAUI interface may need some or all of these fields. In this case, the routing header processor at area 910 d needs to add these going into the switch, and to remove them leaving the switch. To reduce the number of bytes that need to be sent over XAUI, these three fields may be removed (if the XAUI interface allows it). In this case, the routing header processor at area 910 b will need to strip these going into the switch, and add them back leaving the switch.

The routing frame header processor receives an Ethernet frame from a MAC, sending a routing frame to the switch. It also standardizes the preamble, start of frame, and inter-frame gap fields, prepends a routing header, and receives a routing frame from the switch, sending the Ethernet frame into a MAC. This processor then strips the routing header and standardizes the preamble, start of frame, and inter-frame gap fields. Note that all frames that are flowing within the fabric are routing frames, not Ethernet frames. The Ethernet frame/routing frame conversion is done only as the packet is entering or leaving the fabric via a MAC. Note also that the routing logic within the switch may change fields within the routing frame. The Ethernet frame is never modified (except the adding/removing of the preamble, start of frame, and inter-frame gap fields).

The routing frame is composed of the routing frame header plus the core part of the Ethernet frame, and is structured as shown in Table 1, below:

TABLE 1 Routing Frame Header Ethernet Frame Packet RF MAC MAC Ethertype/ (data and CRC32 Header destination Source Length padding)

Note that the implementation assumptions for bit sizing are 4096 nodes→12 bit node IDs. These fields may be resized during implementation as needed.

The routing frame header consists of the fields shown in Table 2, below:

TABLE 2 Width Field (Bits) Notes Domain ID 5 Domain ID associated with this packet. 0 indicates that no domain has been specified. Mgmt Domain 1 Specifies that the packet is allowed on the private management domain. Source Node 12 Source node ID Source Port 2 0 = MAC0, 1 = MAC1, 2 = MAC_management processor, 3 = MAC_OUT Dest Node 12 Destination node ID Dest Port 2 0 = MAC0, 1 = MAC1, 2 = MAC_management processor, 3 = MAC_OUT RF Type 2 Routing Frame Type (0 = Unicast, 1 = Multicast, 2 = Neighbor Multicast, 3 = Link Directed) TTL 6 Time to Live - # of hops that this frame has existed. Switch will drop packet if the TTL threshold is exceeded (and notify management processor of exception). Broadcast ID 5 Broadcast ID for this source node for this broadcast packet. Checksum Checksum of the frame header fields. Total 46 +checksum

If a switch receives a packet that fails the checksum, the packet is dropped, a statistic counter is incremented, and the management processor is notified.

The routing frame processor differentiates between several destination MAC address encodings. As a reminder, MAC addresses are formatted as shown in FIG. 5 b. The following table describes the usage of the 3 byte OUI and 3 byte NIC specific field within the MAC address. One of the novel aspects of the system and method disclosed herein is the use of additional address bits to encode an internal to external MAC mapping, as shown also in the Table 3, below, in the second entry under “Fabric Internal Node local address Hits MAC Lookup CAM”.

TABLE 3 MAC Address Type 3 bytes OUI 3 bytes NIC Specific Operation External Multicast bit Arbitrary Packet unicast Misses MAC not set routed to Lookup CAM gateway node #. Fabric Internal Arbitrary Node local address (meaning Packet unicast Node local low 2 bits - port unit routed to address ID) are not present. MAC fabric node # Hits MAC Lookup CAM for entry obtained from Lookup CAM marked as Node Local. MAC Lookup CAM Fabric Internal Arbitrary Arbitrary Packet unicast Arbitrary MAC routed to address fabric node # Hits MAC obtained from Lookup CAM MAC Lookup CAM Node Encoded Unicast 10 bits: Packet Unicast Locally SS_MAC_NODE_ENCODED_MAGIC unicast administered 12 bits: Node ID routed to OUI == Switch 2 bits: Port ID Node ID. OUI Link Encoded Unicast 12 bits: Packet sent Unicast Locally SS_MAC_LINK_ENCODED_MAGIC down specific administered 7 bits: Reserved Link #. OUI == Switch 3 bits: Link # (0-4) OUI 2 bits: Port Multicast/ Multicast bit Arbitrary Packet Broadcast set broadcast routed through fabric and gateways. Neighbor Multicast bit 12 bits: Packet sent Multicast set SS_NEIGHBOR_MCAST_MAGIC through all Locally 12 bits: Reserved XAUI links to administered neighboring OUI = Switch nodes and not OUI rebroadcast to other nodes

Further, other novel aspects can be found in Table 3 under “Node Encoded Unicast” as well as “Link Encoded Unicast,” allowing one internal node or link to address all external MAC sections, and the “Neighbor Multicast” entry, allowing a multicast to neighboring nodes.

Note that the values SS_MAC_NODE_ENCODED_MAGIC and SS_MAC_LINK_ENCODED_MAGIC are constant identifiers used for uniquely identifying these MAC address types. The term “magic number” is a standard industry term for a constant numerical or text value used to identify a file format or protocol. These magic numbers are configured in two registers (magicNodeEncodedMAC and magicLinkEncodedMAC that default to standard values during hardware initialization, but allow the management processor software to change them if necessary.

The header processor contains a MAC Lookup CAM (Content Addressable Memory), macAddrLookup, that maps from 6 byte MAC addresses to 12-bit Node IDs, as shown in Table 4, below.

TABLE 4 MAC Lookup CAM Input MAC Lookup CAM Output Node Local MAC Address Node ID Port ID 1 bit 6 bytes 12 bits 2 bits

The number of rows in this CAM is implementation dependent, but would be expected to be on the order of 256-1024 rows. The management processor initializes the CAM with Node ID mappings for all the nodes within the SS fabric. There are two types of rows, depending upon the setting of the Node Local bit for the row. The Node Local field allows a 4:1 compression of MAC addresses in the CAM for default MAC addresses, mapping all four MACs into a single row in the CAM table, which is Table 5, below.

TABLE 5 MAC Address Node Type Local MAC Address Port ID Node 1 A Node Encoded Address refers to a Smooth Stone Taken from Local assigned MAC address for a node. It encodes the port # low 2 bits of (MAC0, MAC1, management processor, Rsvd) into a 2- MAC Address bit Port ID in the lowest two bits of the NIC address Input field. Ignores low 2 bits during match. Arbitrary 0 Matches against all 6 bytes Taken from CAM Output field

The arbitrary rows in the CAM allow mapping of the MAC address aliases to the nodes. Linux (and the MACs) allow the MAC addresses to be reassigned on a network interface (e.g., with ifconfig eth0 hw ether 00:80:48:BA:d1:30). This is sometime used by virtualization/cloud computing to avoid needing to re-ARP after starting a session.

The switch architecture provides for a secondary MAC Lookup CAM that only stores the 3 bytes of the NIC Specific part of the MAC address for those addresses that match the Switch OUI. The availability of this local OUI CAM is determined by the implementation. See Table 6, below.

TABLE 6 MAC Lookup CAM Input MAC Lookup CAM Output MAC Address NIC Specific Node ID Port ID 3 bytes 12 bits 2 bits

The maximum number of nodes limitation for three types of MAC address encodings may be evaluated as follows:

1. Default MAC Addressees—management processor sets Node Local mappings for each of the nodes in the fabric. There is one entry in the CAM for each node. Max # of nodes is controlled by maximum # of rows in the MAC Address Lookup CAM.

2. Node Encoded Addresses—All the MACs are reprogrammed to use Node Encoded Addresses. In this way the Node IDs are directly encoded into the MAC addresses. No entries in the MAC Lookup CAM are used. Max # of nodes is controlled by maximum # of rows in the Unicast lookup table (easier to make big compared to the Lookup CAM). Note that this also gives us some risk mitigation in case the MAC Lookup CAM logic is busted. Provides use case for the node encoded addresses idea.

3. Arbitrary MAC Address Aliases—Takes a row in the CAM. As an example, a 512-row CAM could hold 256 nodes (Node local addresses)+1 MAC address alias per node.

Since the Lookup CAM is only accessed during Routing Header creation, the management processor actually only needs to populate a row if the MAC address within the fabric is being used as a source or destination MAC address within a packet. In other words, if two nodes never will talk to each other, a mapping row does not need to be created. But usually the management processor won't have that knowledge, so it's expected that mappings for all nodes are created in all nodes. Also note that even if an entry is not created in the Lookup CAM, the routing will actually still succeed by routing the packet out the Ethernet gateway, through an external router, back into the Fabric, to the destination node.

Table 7 defines how to set fields within the Routing Header for all the fields except for destination node and port.

TABLE 7 Field Set To Domain ID Set to the macDomainID field for the MAC that the packet came from. Mgmt Set to the macMgmtDomain field for the MAC that Domain the packet came from. Source Node Switch Node ID Source Port Source MAC Port ID RF Type Multicast (if dstMAC multicast and not Neighbor Multicast format) Neighbor Multicast (if dstMAC multicast and is Neighbor Multicast format) Link Directed (is Link Encoded format) Unicast (if not one of the above) TTL 0 Broadcast If dstMAC is unicast - Set to 0 ID If dstMAC is multicast - Set to incremented local broadcast ID (bcastIDNext++ & 0xf)

Table 8 defines how to set destination node and port for addresses within the fabric:

TABLE 8 Field: Field: Destination Destination Case Node Port Node Encoded Dest Address Dest Node Dest Port Link Encoded Dest Address Encoded Link Dest Port Hits Lookup CAM (node local) CAM Dest Node Dest MAC (low 2 bits) Hits Lookup CAM (not node local) CAM Dest Node CAM Dest Port

Table 9 defines how to set destination node and port for addresses outside the fabric:

TABLE 9 Field: Field: Destination Destination Case Node Port Came in an OUT Ethernet, but no Drop packet, update statistics counter secondary gateway defined Came in an OUT Ethernet, and secondaryEthGatewayNode[OUT] OUT secondary gateway defined From an Inside MAC, but no Drop packet, update statistics primary gateway defined counter, and notify management processor From an Inside MAC, and primaryEthGatewayNode[fromPort] OUT primary gateway defined

Additionally, the management processor software architecture of the system and method disclosed here currently depends on the ability of management processor nodes to “trust” each other. This more rigorous security on management processor to management processor communication is desirable, as well a better security on private management LANs across the fabric. This fabric issue may be mitigated by simply defining, for environments that require multiple “hard” security domains, that customers simply don't mix security domains within a fabric. In such cases, it may be possible to connect 14-node boards to the top of rack switch, allowing customers to have VLAN granularity control of each 14-node board.

The multi-domain fabric architecture that has been described addresses the lack of VLAN support by creating secure “tunnels” and domains across the fabric, and it can interoperate with VLAN protected router ports on a 1:1 basis.

The approach to domain management in the system and method disclosed here is as follows: Support multiple domain IDs within the fabric. Allow each of the MACs within a node (management processor, MAC0, MAC1, Gateway) to be assigned to a domain ID individually (and tagged with domain 0 if not set). Allow each of the MACs within a node to have a bit indicating access to the management domain. The domain IDs associated with a MAC could only be assigned by the management processor, and could not be altered by the A9. For frames generated by MACs (both inside and outside), the routing frame processor would tag the routing frame with the domain ID and management domain state associated with that MAC. Domains would provide the effect of tunnels or VLANs, in that they keep packets (both unicast and multicast) within that domain, allowing MACs outside that domain to be able to neither sniff or spoof those packets. Additionally, this approach would employ a five-bit domain ID. It would add options to control domain processing, such as, for example, a switch with a boolean per MAC that defines whether packets are delivered with non-defined (i.e., zero) domain ID, or a switch that has a boolean per MAC that defines whether packets are delivered with defined (non-zero) but non-matching domain IDs. A further option in the switch could turn off node encoded MAC addresses per MAC (eliminating another style of potential attack vector).

To keep management processor to management processor communication secure, the management domain bit on all management processor MACs could be marked. Generally, the management processor should route on domain 1 (by convention). Such a technique allows all the management processor's to tunnel packets on the management domain so that they cannot be inspected or spoofed by any other devices (inside or outside the fabric), on other VLANs or domains. Further, to provide a secure management LAN, a gateway MAC that has the management domain bit set could be assigned, keeping management packets private to the management processor domain. Additionally, the switch fabric could support “multi-tenant” within itself, by associating each gateway MAC with a separate domain. For example, each gateway MAC could connect to an individual port on an outside router, allowing that port to be optionally associated with a VLAN. As the packets come into the gateway, they are tagged with the domain ID, keeping that traffic private to the MACs associated with that domain across the fabric.

The switch supports a number of registers (aka CSRs, aka MMRs) to allow software or firmware to control the switch. The actual layout of these registers will be defined by the implementation. The fields listed in Table 10 are software read/write. All these registers need to have a mechanism to secure them from writing from the A9 (could be secure mode or on a management processor private bus).

TABLE 10 Field Size Notes Adaptive 1 bit Adaptive unicast routing enabled. broadcastLateral 1 bit Enable to have broadcasts go through lateral links, rather than just Up and Down links. Turning this off will work for most topologies and will reduce # duplicate broadcast packets. intPortBroadcastVec 4 bits Vector of ports to send internally generated broadcast packet into. extPortBroadcastVec 4 bits Vector of ports to send externally generated broadcast packet into. linkDir[LINKS] Array [LINKS] x Specifies link direction for each link 2 bits (0 = DOWN, 1 = LATERAL, 2 = UP, 3 = Rsvd) linkState 5 bits Link state vector for each of the 5 links. Bit set indicates that link is active (trained and linked). linkType[LINKS] Array [LINKS] x Specifies type of each link 2 bits (0 = No Link, 1 = XAUI, 2 = Ethernet} localBroadcastM3Snoop 1 bit When set, then we'll always send a copy of the locally initiated broadcast into the management processor. The use case here is where the management processor wants to see the gratuitous ARPs that are locally initiated so that it can communicate across the management processor fabric and add corresponding entries into the local unicast routing tables. macAddrLookup Lookup CAM which is MAC address lookup CAM to convert MAC described elsewhere in addresses to Node IDs. the document macAcceptOtherDomain[MAC] 1 bit[MAC] Defines that the MAC accepts packets that are tagged with a non-zero, non-matching domain ID. macAcceptZeroDomain[MAC] 1 bit[MAC] Defines that the MAC accepts packets that are not tagged with a domain (i.e. 0 domain) macDomainID[MAC] 5 bits[MAC] Defines the Domain ID for each of the 4 MACs. A value of 0 indicates that the domain ID for that MAC is not set. macMgmtDomain[MAC] 1 bit[MAC] Defines that the MAC may access the management domain. Setting this tags the management domain in the routing frame, as well as allows the switch to route management frame packets into this MAC. magicNodeEncodedMAC 10 bits Magic number for Node Encoded MAC addresses magicLinkEncodedMAC 12 bits Magic number for Link Encoded MAC addresses maxTTL 6 bits Maximum TTL count allowed in a routing header. Exceeding this number of hops causes the switch to drop the packet, update a statistic counter, and inform the management processor. myNodeID 12 bits Need not be contiguous. Subtree's should ideally be numbered within a range to facilitate subtree network proxying. myOUI 3 bytes 3 upper bytes of MAC addresses in fabric. Should be the same for all nodes in the fabric. nodeRangeEnable 1 bit Enables the expanded Node ID matching of [nodeRangeLo, nodeRangeHi]. Used for Network Proxying through a subtree. When enabled, a packet will be routed into the node (rather than through the node) if either DstNode == myNodeID OR (nodeRangeLo <= DstNode <= nodeRangeHi). nodeRangeHi 12 bits Enabled with nodeRangeEnable. Specifies high node ID of node range match. nodeRangeLo 12 bits Enabled with nodeRangeEnable. Specifies low node ID of node range match. noFlowControl 1 bit When enabled, there will be no flow control. portRemap[INT_PORTS]; Array [INT_PORTS] x Allows remapping of incoming destination 2 bits port IDs to the internal port where it'll be delivered. This register defaults to an equivalence remapping. An example of where this will get remapped is during Network Proxy where the management processor will remap MAC0 packets to be sent to the management processor. INT_PORTS = 4. Array elements are the Ports enumeration (management processor, MAC0, MAC1, OUT). 2 bits contents is the Ports enumeration. primaryEthGatewayNode[INT_PORTS] Array [INT_PORTS] Specifies Node ID of primary Ethernet of 12-bit gateway for this node. Packets destined to node IDs that aren't within the fabric will get routed here. promiscuousPortVec 4 bits Can be configured for Promiscuous Mode allowing traffic on one or more links to be snooped by the management processor or A9s in order to collect trace data or to implement an Intruder Detection System (IDS). This causes all traffic passing through the switch to be copied to the internal ports defined by this port vector. routeForeignMACsOut 1 bit When enabled, a MAC address that does not contain a myOUI address, will not check the MAC lookup CAM, and will get treated as a MAC lookup CAM miss, thus getting routed to the gateway port. This saves latency in the common case of not populating the CAM with foreign MAC aliases. secondaryEthGatewayNode[INT_PORTS] Array [INT_PORTS] Specifies Node ID of secondary Ethernet of 12-bit gateway. Incoming (from OUT) packets routing through the fabric will be sent here. unicastPortsFromOtherExt 1 bit An incoming unicast from an external Gateways gateway will get the gateway node put into the source node field of the routing header. Upon reaching the destination node, this bit will be checked. When the bit is clear, the external gateway node must match the destination gateway node for it to be delivered to internal ports. This is to handle the case where the fabric is connected to an external learning switch that hasn't yet learned the mac/port relationship, and floods the unicast packet down multiple ports. This will prevent a fabric node from getting the unicast packet multiple times. unicastRoute[NODES] Array [NODES] of Link vector of unicast next route. 10 bits is 2- 10 bits bit weight for each of 5 links.

The registers shown in Table 11 are contained within the Switch implementation, but need not be software accessible.

TABLE 11 Field Size Notes bcastIDNext 5 bits Next broadcast sequence ID to issue next. Hardware will increment this for each broadcast packet initiated by this node. bcastIDSeen[BCAST_ID_LEN] Array [BCAST_ID_LEN] FIFO list of broadcast tags seen by of 5 bits. this node. bcastIDSeenNext # bits to index into Next array position into BCAST_ID_LEN bcastIDSeen[ ] to insert a broadcast tag.

Note that software should be able to update the routing tables (unicastRoute) and the macAddrLookup CAM atomically with respect to active packet routing. One implementation will be to hold off routing access to these tables during an update operation.

Broadcast/Multicast Routing

FIG. 6 shows an exemplary broadcast mechanism 1000 according to one aspect of the system and method disclosed herein. The link between nodes N101001 and N21 1002 is down, as indicated by the dashed line 1003. During routing header generation of multicast packets, the source node puts an incremented broadcast ID for that source node in the routing frame (rframe.bcastID). When a node receives a multicast routing frame (i.e. rframe.rfType==Multicast∥rframe.rfType==NeighborMulticast), it checks to see whether it has already seen this broadcast packet. The check is done by accessing the bcastIDSeen CAM with a tag formed with the broadcast source node and the broadcast ID. If it has already been seen (i.e. CAM hit), no action is be performed. If the broadcast frame has not been seen before, it broadcasts it to appropriate internal ports and external gateways (intPortBroadcastVec register) and rebroadcasts it through all outward XAUI links except for the link it came in on. Note that it only broadcasts through laterals if the broadcastLateral register is set. It is unnecessary to broadcast laterals on most topologies, and doing so may reduce the number of duplicated broadcast packets by disabling it. It then adds this broadcast tag to the bcastIDSeen CAM in FIFO order. In FIG. 7, N04 1004 initiates a broadcast to all neighbors, i.e., N11 1105. N11 has not seen the packet, so it broadcasts to all non-incoming neighbors, which, in this example, are N21 1002, N20 1006, N03 1007, and N05 1008, and accepts the packet internally. Nodes N03 and N05 haven't seen the packet, so they accept the broadcast internally and are done. N21 hasn't seen the packet, so it broadcasts the packet to all active, non-incoming links (e.g., N10, N12 1009), and accepts the packet internally. N20 broadcasts the packet to all active, non-incoming links (i.e., N12), and accepts the packet internally. N10 broadcasts down to N00 1010, N01 1011, and N02 1012. N12 rebroadcasts to N06 1013, N07 1014, N08 1015 and to one of N21 and N20 (the one it didn't get the broadcast packet from). Note that one of N20 and N21, and N12, see the packet twice. They take action only on their first instance, the secondary times it hits the broadcast CAM as a duplicate, and the packet is ignored.

Unicast Routing

Unicast to Other Node

Unicast routing (as shown in FIG. 7) is responsible for routing non-multicast (i.e. unicast) packets to the next node. This is done by utilizing a software computed unicastRoute[ ] next node routing table that provides a vector of available links to get to the destination node.

Condition

rframe.rfType=Unicast

Routing

There are substantial complexities related to routing around faults. Fault free routing and routing around faults will be discussed separately.

Traditionally in tree routing, the packet will be routed upward until a common parent of (source, destination) is reached. This upward routing can be deterministic, oblivious, or adaptive. The packet is then routed downward to the destination using deterministic routing.

As an example, FIG. 7 illustrates a packet routing from node N00 1010 to N08 1015. The packet is routed in the upward phase to the common ancestor (N21) through node N10 1001, and then a descent phase to the destination.

Note that during the upward phase at node N10, there are two candidate links (N10,N21) and (N10,N20). The first candidate link could be chosen deterministically, or an adaptive algorithm could dynamically select either of the links. But, once the node reaches the common ancestor and turns downward, there are no redundant paths (in general) for the node to reach the destination.

Unicast Routing in the Presence of No Faults

Each link is annotated within this unicastRoute table with a 2-bit linkWeight where software can express the relative cost/distance to the destination node via this link. By convention, link weights should represent:

-   -   0=No route     -   3=Direct next-hop connection     -   1 and 2=Software computed relative costs. As an example if there         are routes across 3 links with costs of 2 hops, 3 hops, and 6         hops, the first two links could be assigned weight=2 and the 6         hops path could be assigned weight=1.         Algorithm for fault-free unicast routing:     -   Get link weight vector from the unicast routing table         -   linkWeightVector=unicastRoute[rframe.dstNode]     -   Remove link that it came in on to remove possibility of sending         it back     -   Remove any links that are not up     -   At this point, have a candidate list of links with associated         link weights.     -   Iterate through link weights, starting with highest priority (3)         down through 1. Gather candidate list of links at this priority,         stopping once the candidate list has at least one link. The         result is a candidate list of links at the highest priority. As         an example, if there are 2 links at weight=2, and 2 links at         weight=1, the prioritized candidate list will contain the two         links at weight=2.     -   The adaptive register is checked to determine whether to do         adaptive or deterministic routing.         -   adaptive==0 indicates that deterministic routing is to be             used, so the first link is chosen from the prioritized             candidate list.         -   adaptive==1 indicates that adaptive routing is to be used.             The switch implementation will choose an algorithm for             adaptively choosing the target link from the prioritized             candidate list. This adaptive algorithm could be as simple             as round-robin around the list. Alternatively, may choose to             factor in other attributes e.g. FIFO free depth, link speed,             . . .             -   An implementation option could be to add a register                 option to allow the router to adaptively choose from all                 non-zero weights, or to only adaptively choose from the                 highest priority candidate lists.     -   The packet is sent out the selected link.

Fault-Resilient Unicast Routing

A couple of issues contribute to the complexity of fault-resilient unicast routing:

-   -   The desire to do fault routing with only localized knowledge. A         node implicitly knows that a link is down to a neighbor node. We         choose a design to avoid having to communicate that a link (or         node) goes down elsewhere in the fabric due to the complexities         of maintaining a global, unified state in the presence of         failures.     -   The nature of routing in a tree. During the ascent phase of         packet routing, links can be adaptively chosen from redundant         links so it can be straightforward to avoid a link with the         normal adaptive link selection.     -   But, once the packet starts descending, traditionally there is         not redundant paths for the descent path (that follow the         routing rules), so fault routing can become challenging.     -   FIG. 8 illustrates a link failure (N10,N21) and unicast routing         selected the (N10, N20) link using the normal adaptive routing         algorithm previously described. But note, if the packet is         routed up to N20 and link (N20,N12) is down, it has no easy path         to get to the destination.

We have two approaches to handling routing around fails:

-   -   Software can compose alternative but non-desirable routes with         weight=1. We'll call these escape routes. These are low priority         routes that may violate the strict routing rules used during         routing around faults. As an example, if the link (N20, N12) was         down, the unicastRoute[N08] entry for N20 could contain link to         N12 with weight=2 and a link to N11 with weight=1. In this way,         the normal adaptive routing algorithms will automatically do the         N20->N11->N21->N12->N08 path.     -   The fabric architecture includes a technique that we refer to as         “misrouting”. Misrouting provides for iterative backtracking     -   Both of these techniques will provide substantial unicast         fault-resilience.

Unicast Misrouting

As an example, consider the following topology, with 3 links 1101, 1102 and 1103 that have failed (shown in Red in FIG. 9). Consider a unicast route from N0 to N3. We'll consider the following routing to understand the misrouting technique, understanding that this is only one of several routes that could have been chosen adaptively.

-   -   Packet routed N0 to N6.     -   Packet routed N6 to N10     -   N10 sees that it has no paths to get to N3, other than the link         it came in on. N10 sets the misrouting bit in the routing         header, and sends it back to N6.     -   N6 sees that the packet is being misrouted, sets the bit for the         N10 link in the misrouteVector in the routing header, chooses an         alternative link that has not been misrouted, and sends the         packet to N11.     -   N11 sees that it has no path to N3, other than the link it came         in on. misrouting bit is already on, and sends it back to N6.     -   N6 sees that the packet is being misrouted, adds N11 link to the         misrouteVector (now contains N10 and N11 link IDs), chooses an         alternative link that has not been misrouted, and sends it N7.     -   N7 sees that the misrouting bit is set, but does have a valid         link to N3 (to N12), and thus clears the misrouting bit in the         header, and forwards the packet to N12.     -   N12 sends to N9.     -   N9 unicastRoute now likely contains link to N3 (weight=3) and         link to N8 (weight=2). Normal adaptive routing will not choose         the direct link to N3 since it's down, and will route the packet         to N8, then finally to N3.     -   If N6 had exhausted its list of candidate links (meaning the         misrouteVector masked them all), the implementation then has two         choices:         -   drop the packet and inform the M3 of the failure to route.         -   clear the misrouteVector leaving misrouting set, and forward             the packet through one of the downward facing links (if one             exists). This will retry misrouting at one layer lower. The             implementation may want to have a register bit             (enableRecursiveMisrouting) to enable this retry at lower             layer option.

There is a register enableMisrouting that allows software to control whether the switch will initiate the misrouting algorithm.

Multi-Domaining

Also known to the inventors is Multi-Domaining, whose goal is to increase the addressability of nodes to a large number of nodes (e.g., 64K nodes), without having to increase the size of the unicast routing table to 64K nodes.

As currently described, the unicast routing table is a single-dimension array indexed by node number (i.e. 0 to MAX_NODES−1), where a typical implementation will be between 256 and 4K nodes.

This section will now describe how the current architecture is altered to support multiple domains, with 64K max nodes.

-   -   The node namespace is changed from a node ID from 0 to         MAX_NODES−1, to a 2-tuple of (domain ID, node ID), where both         domain ID and node ID range from 0 to 255. So, there can         effectively be 256 domains where each domain can contain up to         256 nodes.     -   The unicast routing table is changed from a single dimension         table of size MAX_NODES, to a two-dimension table of size 256.         The unicast routing table is now changed from a structure of         unicastRoute[NODES] to unicastRoute[2][256].         -   Local domain routing: When routing to a node within this             domain, the unicast routing table is accessed as             unicastRoute[0] [node ID], and provides a weighted link             vector to route to the specified node ID from the current             node.         -   Remote domain routing: When routing to a node within a             remote domain, the unicast routing table is accessed as             unicastRoute[1][domain ID], and provides a weighted link             vector to route to the specified domain ID from the current             node.     -   Routing frame: One bit is added to the routing frame, dstRemote,         which is set true when routing to a remote domain.         -   Locally administered MAC addresses: The section below             describes the Node Encoded Unicast MAC address encoding as             follows:

Node Unicast 10 bits: Encoded Locally SS_MAC_NODE_ENCODED_MAGIC Unicast administered 12 bits: Node ID OUI == Switch 2 bits: Port ID OUI

This gets altered for multi-domaining as follows:

Node Unicast 6 bits: Encoded Locally SS_MAC_NODE_ENCODED_MAGIC Unicast administered 8 bits: Domain ID OUI == Switch 8 bits: Node ID OUI 2 bits: Port ID

-   -   Creating the routing frame header: Table 2 describes the         algorithms for creating the routing frame header. This is         augmented in the multi-domaining case by:

if ( dstDomain == myDomainID ) { // Route to local domain rframe.dstRemote = false; rframe.dstNode = dstNode; } else { // Route to remote domain rframe.dstRemote = true; rframe.dstNode = dstDomain;

Network Proxy

The concept of network proxy is the ability of the main processors (FIG. 5A, 905) to maintain network presence while in a low-power sleep/hibernation state, and intelligently wake when further processing is required. There are several architectural features related to Network Proxy:

-   -   There is a CSR (portRemap) to allow the remapping of Port IDs.         In effect, when the switch is to deliver a packet to an internal         MAC0 port (e.g. FIG. 5A, 902), this Port Remapping CSR allows         software to remap MAC0 to the management processor MAC (e.g.         FIG. 5A, 907) and have the packet delivered to the management         processor for Network Proxy processing. This remapping CSR could         also be used to remap MAC1 traffic to MAC0, or MAC1 traffic to         the management processor.     -   Normally, the switch looks at the destination node ID of the         routing frame to decide whether the packet is delivered to an         internal port within the node, or gets routed to other XAUI         connected nodes. This is done by matching Destination Node ID to         “My Node ID”. The Node ID Match register (nodeRangeLo,         nodeRangeHi) causes the packet to be delivered to an internal         port within the node if         nodeRangeLo<=Destination_Node<=nodeRangeHi∥myNodeID==Destination_Node.         This allows a node to proxy for a subtree of nodes.         A typical use sequence would be of the form:     -   Management processor maintains the IP to MAC address mappings         for MAC0 and MAC1 on the node. This can be done via either         explicit communication of these mappings from the main processor         OS to the management processor, or can be done implicitly by         having the management processor snoop local gratuitous ARP         broadcasts.     -   The main processor coordinates with the management processor to         go to a low power dormant state. During this transition, the         management processor sets up the Port ID remapping CSR to route         MAC0 and MAC1 traffic to the management processor.     -   The management processor processes any incoming MAC0/MAC1         packets. There are 3 categories of processing:         -   Respond to some classes of transactions that require simple             responses (e.g. ARP responses and ICMP ping).         -   Dump and ignore some classes of packets, typically unicast             or broadcast packets that are targeting other computers.         -   Decide that the main processor must be woken to process some             classes of packets. The management processor will wake the             main processor, undo the Port ID remapping register, and             re-send the packets back through the switch where they will             get rerouted back to MAC0/1.

Wake-on-LAN Magic Packet

In a traditional desktop computer, the computer to be woken is shut down (sleeping, hibernating, or soft off; i.e., ACPI state G1 or G2), with power reserved for the network card, but not disconnected from its power source. The network card listens for a specific packet containing its MAC address, called the magic packet, broadcast on the broadcast address for that particular subnet (or an entire LAN, though this requires special hardware or configuration). The magic packet is sent on the data link or layer 2 in the OSI model and broadcast to all NICs within the network of the broadcast address; the IP-address (layer 3 in the OSI model) is not used. When the listening computer receives this packet, the network card checks the packet for the correct information. If the magic packet is valid, the network card takes the computer out of hibernation or standby, or starts it up.

The magic packet is a broadcast frame containing anywhere within its payload: 6 bytes of ones (resulting in hexadecimal FF FF FF FF FF FF), followed by sixteen repetitions of the target computer's MAC address. Since the magic packet is only scanned for the string above, and not actually parsed by a full protocol stack, it may be sent as a broadcast packet of any network- and transport-layer protocol. It is typically sent as a UDP datagram to port 0, 7 or 9, or, in former times, as an IPX packet.

Using the Network Proxy architecture just described, the management processor can support these Wake-On-LAN packets. It will get these broadcast packets, will know the MAC addresses for the other MACs on the node, and be able to wake up the main processor as appropriate. No further functionality is needed in the switch to support these Wake-on-LAN packets.

While the foregoing has been with reference to a particular embodiment of the invention, it will be appreciated by those skilled in the art that changes in this embodiment may be made without departing from the principles and spirit of the disclosure, the scope of which is defined by the appended claims. 

1. A switch fabric system, comprising a plurality of nodes; a plurality of links associated with each node that connect the node to another node in the plurality of nodes to create one of a tree topology and a graph topology of the switch fabric for routing data through the plurality of nodes; and wherein each link is designated as one of an Up link, a Down link and a Lateral link within the topology.
 2. The switch fabric system of claim 1, wherein each node may be one of a computational and switch node and a switch node.
 3. The system of claim 2, wherein each node includes input/output.
 4. A switch fabric system having a plurality of nodes wherein each node may be one of a computational and switch node and a switch node.
 5. A switch for a switch fabric system having a plurality of nodes with a segmented media access control (MAC) architecture, the switch comprising: at least one processor unit that controls routing of data in the switch; a first portion of the switch connected to the at least one processor and having an interface to an Ethernet controller for data; a second portion of the switch that converts the data into a plurality of packets and sends the plurality of packets to a fabric switch; and the packet switch having a port that is connected to the second portion of the switch that perform Ethernet signaling.
 6. The system of claim 1, wherein the switch fabric further comprises at least one processor unit that controls routing of data in the switch, a first portion of the switch connected to the at least one processor and having an interface to an Ethernet controller for data, a second portion of the switch that converts the data into a plurality of packets and sends the plurality of packets to a fabric switch, and the packet switch having a port that is connected to the second portion of the switch that perform Ethernet signaling.
 7. The system of claim 4, wherein the switch fabric further comprises at least one processor unit that controls routing of data in the switch, a first portion of the switch connected to the at least one processor and having an interface to an Ethernet controller for data, a second portion of the switch that converts the data into a plurality of packets and sends the plurality of packets to a fabric switch, and the packet switch having a port that is connected to the second portion of the switch that perform Ethernet signaling.
 8. A switch fabric system, comprising a plurality of nodes; each nodes having a management processor; and wherein a communications path between the management processors of the plurality of nodes is secure so that the management processors can trust each other.
 9. A method of re-purposing an Ethernet MAC controller IP for inside MACs and outside MACs, and leveraging what would normally be the physical signaling for the MAC to feed into the switch.
 10. A method of nonspoofing communication in a switch fabric system, the method comprising: providing a plurality of nodes wherein each node has a management processor and one or more pieces of software being executed by the management processor are verified software; and establishing a trusted relationship between the plurality of nodes, wherein establishing the trusted relationship further comprises inserting a domain ID into a routing header at a source node of a packet; and communicating securely between the management processors of the source node and a destination node, wherein the secure communication occurs when the routing header of the packet with the domain ID is verified by the destination node.
 11. A method of fault-resilient unicast routing in a switch fabric having a plurality of nodes and a plurality of links associated with each node that connect the node to another node in the plurality of nodes to create a switch fabric with a plurality of routes, the method comprising: generating an escape route from a first node to a second node in the switch fabric wherein the escape route has a low priority weight; and misrouting data from the first node to the second node when a link is inactive wherein the escape route and misrouting provide fault tolerance to the switch fabric.
 12. The method of claim 11, wherein the misrouting data from the first node to the second node further comprises iteratively backtracking to route data from the first node to the second node through one or more intervening nodes when a link between the first and second nodes is inactive.
 13. The method of claim 12, wherein the iteratively backtracking further comprises: setting, when a node in the data path does not have a link path to the second node, a misrouting bit in a header of the data; sending the data back to an originating node that sent the data to the node that set the misrouting bit; choosing one or more alternate links, by the originating node, for the data; and clearing the misrouting bit in a header of the data if the data reaches the second node over one of the alternative links. 14-24. (canceled) 